1. Field of the Invention
The present invention relates to an LED driver. More particularly, the present invention relates to an LED driver, which provides improved power efficiency and light efficiency, and generates a drive current without distortion.
2. Description of the Related Art
Light emitting diodes (LEDs) are used as a light source of a liquid crystal display (LCD) apparatus, as well as a digital micromirror device (DMD) display apparatus such as a digital light processing (DLP) projection TV, a projector, and the like using a digital micromirror device (DMD).
FIG. 1 illustrates the DMD display apparatus which employs LEDs as the light source. The DMD display apparatus employs a plurality of LED. modules 210 corresponding to respective red, green and blue colors of and RGB signal.
The LED modules 210 are driven by an LED driver 200, and emit light signals of red, green and blue to sequentially project them to a DMD module 230 through a lens 220. A large number (hundreds of thousands or millions) of mirrors 240 are integrated in the DMD module 230 by a micro electromechanical systems (MEMS) process, and independently turn on/off. Accordingly, RGB color signals projected to the DMD module 230 form a predetermined picture on a screen 250.
LEDs have several advantages over conventional discharge lamps and the like. Referring to a wave form 254 illustrated in FIG. 2, the DMD display apparatus using LEDs as the light source has high usability of the light source as compared with a moving wave form 252 of a conventional display apparatus using a discharging lamp as the light source. Thus, the DMD display apparatus has high light efficiency. LEDs also have a longer life span than the discharging lamp, and a display apparatus that employs LEDs as a light source advantageously has a semi permanent life span since it does not require a mechanical apparatus such as a color wheel.
The LED driver 200 for driving the LED modules 210 typically comprises a circuit configuration as shown in FIG. 3. The LED driver 200 in FIG. 3 may be referred to as a linear mode driving circuit. The LED driver 200 comprises a current detector to detect a current flowing in the LED modules 210; an error amplifier 262 to compare a voltage corresponding to a detected current value and a reference voltage Vref and output an error signal; and an output transistor 264 to increase/decrease a current Io flowing in the LED modules 210 according to the error signal.
The LED driver 200 in FIG. 3 detects the current Io flowing in the LED modules 210, compares the voltage corresponding to the detected current value and the reference voltage Vref, and adjusts the current Io flowing in the LED modules 210 until the two voltages are identical so that a current corresponds to the reference voltage Vref set up in the LED modules 210. In this manner, the amount of current delivered to the different colored LEDs can vary according to their color.
As the amount of light for each of the RGB colors is different in white light, the value of the current Io flowing in the LED modules 210 is different for each of the red, green and blue colors, and it is adjusted through the reference voltage Vref.
The LED driver 200 in such a linear mode displays superior picture quality and light efficiency as it does not generate a ripple in the current Io. It also advantageously achieves a fast transient phenomenon and fast switching. Meanwhile, the LED driver 200 has low power efficiency and large heat generation due to a voltage drop in the output transistor 264. For example, when the driving current is larger than 20 A, power loss of around 200 W may occur.
That is, the LED driver 200 consumes a lot of power due to low power efficiency, thereby putting too much burden on a power supply apparatus and requiring large design capacity. Also, the LED driver 200 requires a large-sized heat radiation plate for heat radiation, thereby increasing size and weight thereof and preventing a compact product design.
The LED driver 200 may also comprise a circuit configuration as shown in FIG. 4, and such a driving circuit may be referred to as a switch mode driving circuit. The LED driver 200 in switch mode typically comprises a current detector, an error amplifier 272, a PWM modulator 274, a gate circuit 276, a switch 278, an inductor 280, a diode 282 and a switch block 284.
The current detector and the error amplifier 272 of the LED driver 200 in the switch mode operate similar to the driving circuit in the linear mode in FIG. 3. The PWM modulator 274 compares an output of the error amplifier 272 and a triangular wave, and generates a pulse width modulation (PWM) signal. The gate circuit 276 drives the switch 278, which is preferably a metal-oxide semiconductor field effect transistor (MOSFET), by the pulse width modulation (PWM) signal. The inductor 280 integrates a square wave pulse output from the switch 278 and allows the LED modules 210 to be supplied with a direct current.
The switch mode LED driver 200 achieves more than 90% power efficiency and has fewer problems in power consumption and heat radiation than the linear mode driving circuit shown in FIG. 3. However, the picture quality thereof is not better than the linear type due to a ripple current caused by the switching, and it has a low light efficiency as a transient phenomenon is slow when switching from one LED module to another LED module.
Accordingly, there is a need for an improved LED driver having high power efficiency and light efficiency, while minimizing a ripple current to improve picture quality.